The present invention relates to a method for measuring an optical system comprising a projection lens, a computer program product for operating a measuring system for an optical system, and a microlithography projection exposure apparatus comprising a computer system comprising a computer program product of this type. Furthermore, the invention relates to a method for monitoring an optical system comprising a projection lens and a method for correcting an optical system comprising a projection lens.
Microlithography projection exposure apparatuses serve for producing microstructured components using a photolithographic method. In this case, a structure-bearing mask, the so-called reticle, is illuminated with the aid of a light source unit and an illumination optical unit and is imaged onto a photosensitive layer with the aid of a projection optical unit. For this purpose, the structure-bearing mask is arranged in an object plane of the projection lens and the photosensitive layer is arranged at the location of an image plane of the projection optical unit. In this case, the light source unit makes available a radiation which is directed into the illumination optical unit. The illumination optical unit serves for making available at the location of the structure-bearing mask a uniform illumination with a predetermined angle-dependent intensity distribution. For this purpose, various suitable optical elements are provided within the illumination optical unit. The structure-bearing mask illuminated in this way is imaged onto a photosensitive layer with the aid of the projection optical unit. In this case, the minimum feature size which can be imaged with the aid of a such a projection optical unit is influenced by various factors.
Firstly, the smaller the wavelength of the radiation used, the smaller the structures which can be imaged. For this reason, it is advantageous to use radiation having the wavelength of 5 nm to 15 nm.
Secondly, it is necessary for the optical elements of the illumination optical unit and/or projection optical unit to be manufactured and positioned highly precisely. Even a small deviation in the position or the surface form from the desired values leads to an impairment of the imaging quality.
Furthermore, impairments of the imaging quality can occur as the operating duration of the microlithography projection exposure apparatus increases. This is caused, for example, by degradation of layers on optical elements, contaminations (that is to say deposits of impurity particles) on surfaces of an optical element of the optical system, but also as a result of deformations of optical elements as a result of the long-term loading with radiation from the light source unit (compaction of lens element and/or mirror materials).
In addition, fluctuations in the quality of the radiation which is provided by the light source unit can also occur as well. Disturbances in the light source unit lead, for example, to a changed intensity distribution and/or angular distribution at the entrance of the illumination optical unit. As a result of this, the imaging mask in the image plane is not illuminated as uniformly as desired or is not illuminated with the required angular distribution, and so the imaging quality is impaired as a result of this as well.
On account of the multiplicity of optical elements from which the optical system of the microlithography projection exposure apparatus is constructed, and on account of the multiplicity of disturbances described above, it is difficult to deduce from an impairment of the imaging quality the optical element at which a disturbance is present and what disturbance has occurred.